1. Field of the Invention
The present invention relates to a data communication system comprising a packet switching system and a packet concentrator in a packet network.
2. Description of the Prior Art
Typical examples of a multiplexing system for packet switching equipment are described in the CCITT RED BOOK X.25 (a user-network interface) and X.75 (a network-network interface). These examples are also briefly described on pp.100-102 of Packet Switching Technology and Its Application (the Institute of Electronic, Information, and Communication Engineers). For a conventional packet concentrating system, there are known two different concentrating systems as described on pp.88 and 89, and pp.102-105 of the above-mentioned Packet Switching Technology and Its Application.
The first concentrating system is shown, for example, in FIG. 31 wherein a packet multiplexer (PMX) 604 in which data 601-1 to 601-3 is sent from lines connected to a plurality of non-packet mode terminals (NPT) 600-1 to 600-3. The data is packed by the PMX and then sent on the same line 605 with terminal identifications in the labels of the packets 606-1 to 608-n.
The second concentrating system is shown, for example, in FIG. 32 wherein a packet concentrator 620 (comprised of elements 620-1 to 620-l) and a packet distributor 630 operate so that the output from each concentrator element is provided to the distributor 630 through an n number of output lines (612-1 to 612-n) which is less than an m number of input packet lines (611-1 to 611-m). Here, a logical channel for the terminal data must be set link by link and different logical channels are set between a terminal 610 and the packet concentrator 620 and between the packet concentrator 620 and packet distributor 630.
Problems to be Solved by the Invention
In the first conventional concentrating system mentioned above, input lines connected to the concentrator 604 do not provide any packet multiplexing because they come from only non-packet mode terminals. Therefore, only a single call can be handled per terminal. The lack of efficiency for this system makes it highly undesirable for large or dense data communication systems.
In the second conventional concentrator system mentioned above, since an individual call is set link-by-link, it becomes necessary for the concentrator to also switch logical channels. Consequently, the concentrator is required to administrate logical channels, i.e., to have a capability to terminate a control signal and execute call processing. It becomes impossible for any switching equipment which is placed higher than the concentrator to identify the sender of information being transmitted because the logical channel number is changed by the time that switching equipment receives the data signal. The concentrator, besides completing its primary concentrating tasks, is therefore required to also execute extensive controlling processes with regard to the sender such as a tariff control and others. The individual packet switching equipment needs the capability of processing functions to terminate a call control signal as a concentrating node for concentrating multiplexed lines by logical channel numbers. This extensive control processing becomes a substantial problem to be solved with the continued expansion of packet line converging and multiplexing of communication media. The conventional system in which a packet switching equipment is arranged with such a concentrating node is costly, and raises a serious problem in that respect.
A need exists for a system that permits the switching equipment to perform the substantive portion of call processing while providing a simpler and less costly concentration of terminal cells.